Solid propellants have numerous advantages over liquids for missile propulsion and as gas generators. Among these are greater safety in storage, handling, and transport, higher density, and simplicity of propellant packaging. Liquid propellants, however, have traditionally offered the huge advantage of ease of throttling and can be extinguished and reignited at will, thereby offering better energy management with minimal waste of on-board propulsive resources.
One application for this invention is in the Divert and Attitude Control Systems (DACS) for kinetic-kill missile warheads. DACS provide for control of vehicles flying outside the earth's atmosphere. Missile DACS typically use solid-propellant gas generators (SPGG) and pintle controlled nozzles to provide propulsive jets that accelerate and point the vehicle in the vacuum conditions of near-earth space. In the typical existing system, the gas generator is ignited at the start of the control period and continues to burn generally at a measured constant rate for the control duration. This is despite the fact that most of the gas is vented uselessly, because control is needed during only about 30% of the flight. As the flight time increases, the necessary control time remains essentially constant. This means that for longer flight times the wasted fuel increases to over 90%.
To mitigate this problem, the current state-of-the-art solid DACS utilizes pintle controlled nozzles. These pintle nozzles vary the size of their throat areas to collectively control the system chamber pressure thereby increasing or decreasing the burn rate of the propellant and hence the thrust. Pintle nozzles can also be used to extinguish the propellant, and thus save propellant. Unfortunately, pintle nozzles have several drawbacks: they add significant inert weight to a system with an already poor mass fraction, they reduce nozzle efficiencies thereby reducing performance, during the extinguishment event they create thrust spikes by opening the nozzle throats, and they can have significant thruster misalignment problems due to the necessity of their being mounted on the outside of the chamber.
Additionally, a pintle nozzle control system can only correct upon measured chamber pressure by varying the rocket motor throat area. Due to propellant ballistic burn rate errors at controlled pressures, resulting thrust has inherent inaccuracies that can only be corrected by the vehicle flight control system as measured through inertial sensors.
A need remains in the art, therefore, for a way to control a solid propulsion system that duplicates the ease of throttling and extinguishment and thrust accuracy of liquid propellants without the use of expensive, heavy and inefficient pintle nozzles. This capability would eliminate the biggest single deficiency of the solid-propellant systems and would have widespread system level improvements including: Mass fraction increasing, Performance Increase, Cost decrease and thrust accuracy increase.